At least 24 people have been killed and more than 200 injured in the centre of Cairo after a protest over an attack on a church erupted into the worst violence since the 18-day uprising that ousted Hosni Mubarak as president of Egypt in February.

Trouble began when a demonstration against the attack in southern Egypt was reportedly met by gunfire close to the state television building.

Fighting spread to Tahrir Square and surrounding streets. Hospitals where the wounded were being treated also came under attack.

State television announced that a curfew was being imposed on the city's downtown area and Tahrir Square, the epicentre of the February uprising that overthrew the former president. The curfew would last from 2am to 7am (midnight to 5am GMT) on Monday.

Appealing for calm after more than 1,000 security force personnel were deployed, interim prime minister Essam Sharaf said: "What is taking place are not clashes between Muslims and Christians but attempts to provoke chaos and dissent."

"The only beneficiary of these events and acts of violence are the enemies of the January revolution and the enemies of the Egyptian people, both Muslim and Christian"

Coptic Christians make up approximately 10% of the Egyptian population, and some have been fearful that Egypt's ongoing political turmoil could allow ultra-conservative Islamists to flex their muscles and inspire a crackdown on social minorities. There has also been criticism of the army for being too lenient on previous attacks against Christians, with many witnesses accusing soldiers of being actively complicit in last night's bloodshed.

Egyptian troops are among the dead following the violence, which comes after several outbreaks of sectarian tensions this year

"We were marching peacefully," said Talaat Youssef, a 23-year-old Christian trader. "When we got to the state television building, the army started firing live ammunition," he added.

Another protester, Essam Khalili, said: "Thugs attacked us and a military vehicle jumped over a sidewalk and ran over at least 10 people," he said.

Online, Egyptians queued up to blame the ruling military council for fomenting the violence, accusing army generals of using social instability as an excuse for cracking down on freedom of expression. "Let there be no doubt, today's killings are committed by #SCAF [the ruling military council]. They are the killers," wrote one Twitter user.

State television put the number injured in the violence at 150, saying three of those killed were soldiers.

More than four vehicles were set on fire and TV footage showed protesters breaking windows of parked cars and army personnel carriers driving towards crowds.

In May, twelve people were killed in clashes between Christians and Muslims after rumours that Christians were holding a woman who had converted to Islam. The incident led the country's ruling military council to order the drafting of new laws to criminalise sectarian violence.

• This article was amended on 10 October 2011. The original headline referred to the violence in Cairo as 'riots'. This has been changed to more accurately reflect the story.